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Cast Iron Is the Name Given to Those Ferrous Metals Containing More Than 1.7% Carbon

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Cast Iron Is the Name Given to Those Ferrous Metals Containing More Than 1.7% Carbon INTRODUCTION Cast iron is the name given to those ferrous metals containing more than 1.7% carbon. Cast irons like steels, are basically alloys of iron and carbon. Cast irons contain more than 2 percent carbon. Most commercially manufactured types are in the range of 2.5 to 4 percent carbon. CAST IRON SKILLET ASTM-888 CAST IRON PIPE COMPOSITION All cast irons contain more than 2% C. Cast iron is the alloy of carbon with 1.7 to 4.5% carbon and 0.5 to 3% silicon. But in some alloy it has Manganese 0.5 to 1.0%, Phosphorous 0.1 to 0.9 %, & Sulphur 0.07 to 0.10%. STRUCTURE It have a matrix structure which may be established either during cooling from the molten state ( as cast condition) or as a result of heat treatment. The main component of cast iron are Iron, carbon, silicon. TYPES OF CAST IRON The best method of classifying cast iron is according to metallographic structure. There are four variables to be considered which lead to the different types of cast iron. Some variables may control the condition of the carbon and also its physical form. The types of cast iron are as follows: 1. White Cast Iron 2. Malleable Cast Iron 3. Gray Cast Iron 4. Chilled Cast Iron 5. Nodular Cast Iron 6. Alloy Cast Iron 1. WHITE CAST IRON It is the cast iron in which all the cast iron in the combined form as cementite. All white cast iron are hypoeutectic. In the figure the alloy at “x” exists as a uniform liquid solution of carbon dissolved in liquid iron. Solidification now begins by the formation of austenite crystals containing about 1 percent carbon. 1. WHITE CAST IRON STRUCTURE: The typical microstructure of white cast iron, consisting of dendrites of transformed austenite (pearlite) in a white interdendritic network of cementite, Higher magnification of the same sample reveals that the dark areas are pearlite. 1. WHITE CAST IRON PROPERTIES: It is hard, brittle & interstitial compound. Due to large amount of cementite it is hard & brittle but extremely brittle & difficult to machine The range mechanical properties for unalloyed white irons is as follows: hardness Brinell 375 to 600, tensile strength 20,000 to 70,000 psi, compressive strength 200,000 lo 250,000 psi and modulus of elasticity 24 to 28 million psi. 1. WHITE CAST IRON APPLICATION: It can be used as liners of cement mixers, ball mills, drawing dies and extrusion nozzles. NOZZLE BALL MILLS CEMENT MIXERS 2. MALLEABLE CAST IRON INTRODUCTION: It is the cast iron in which most or all of the carbon is uncombined in the form of irregular round particles known as temper carbon. This is obtained by heat treatment of white cast iron. 2. MALLEABLE CAST IRON MANUFACTURING: White irons suitable for conversion to malleable iron are of the following range of composition: Carbon 2.00-2.65% Silicon 0.90-1.40% Manganese 0.25-0.55% Phosphorous Less than 0.18% Sulphur 0.05% 2. MALLEABLE CAST IRON Commercially, It can be prepared by two steps: (1) First stage annealing. (2) Second stage annealing. In the first-stage annealing, the white-iron casting is slowly reheated to a temperature between 1650 ˚F and 1750 ˚F. This graphitization starts at the malleableizing temperature. 2. MALLEABLE CAST IRON The initial precipitation of a graphite nucleus depletes the austenite of carbon, and so more is dissolved from the adjacent cementite. The graphite nuclei grow at approximately equal rates in all directions and ultimately appear as irregular nodules or spheroids usually called temper carbon. As shown in figure. MALLEABLE IRON, UNETCHED. IRREGULAR NODULES OF GRAPHITE CALLED TEMPER CARBON. (1OOX) 2. MALLEABLE CAST IRON • After first-stage annealing, the castings are cooled as rapidly as practical to about 1400oF in preparation for the second stage of the annealing treatment. • During the slow cooling, the carbon dissolved in the austenite is converted to graphite on the existing temper-carbon particles, and the remaining austenite transforms into ferrite. After cooling to room temperature the structure is known as standard malleable cast iron. Ferritic malleable iron, temper carbon (black) in a ferrite matrix. Etched in 5 percent nital, 100x, 2. MALLEABLE CAST IRON The changes in microstructure during the malleableizing cycle are shown schematically in Figure. The changes in microstructure as a function of the malleableizing cycle-resulting in temper carbon in a ferrite matrix' 2. MALLEABLE CAST IRON PROPERTIES: When copper is added it increase its tensile strength, corrosion resistance e.t.c. The properties of malleable cast iron are: 1. Tensile Strength 58000-65000 (psi) 2. Yield point 40000-450000 (psi) 3. Elongation % in 2 in 15-20 4. BHN 135-155. 2. MALLEABLE CAST IRON: APPLICATION: It can be widely used as: Pipe fittings. Chain-hoist assemblies. Railroad equipment. Also as industrial casters e.t.c. 3. PEARLITIC MALLEABLE CAST IRON If a controlled quantity of carbon, in the order of 0.3 to 0.9 percent, is retained as finely distributed iron carbide, an entirely different set of mechanical properties results. If manganese is added, the regular \ cycle can be maintained to retain combined carbon throughout the matrix. The second-stage annealing of the normal process may be replaced by, a quench, usually air, which cools the casting through the eutectoid range; fast enough to retain combined carbon throughout the matrix. The matrix is completely pearlitic shown in figure. MATRIX OF PEARLITIC MALLEABLE IRON 3. PEARLITIC MALLEABLE CAST IRON If the cooling rate through the critical range is not quite fast enough to retain all the combined carbon, the areas surrounding the temper- carbon nodules will be completely graphitized, while those at greater distance from the nodules will be pearlitic . Because of its general appearance, this is referred to as a bull’s-eye structure. As shown in figure. BULL’S EYE STRUCTURE 3. PEARLITIC MALLEABLE CAST IRON PROPERTIES: We can also add some alloying element like Manganese, sulphur & copper. Copper may be added to increase corrosion resistance & graphite distribution. If it is heated at 1200F to 1300F it improves its machineability, toughness & lower the hardness. The spheroidize the pearlite as shown in figure. Microstructure of a pearlitic malleable iron tempered to obtain a spheroldite matrix. Nital etch, 500x 3.PEARLITIC MALLEABLE CAST IRON APPLICATION: It can widely used in industries application like: Camshafts and crankshafts in automobiles Rolls, pumps, nozzles, cams, and rocker arms as machine parts. For a variety of small tools such as wrenches, hammers, clamps, and shears. CAMSHAFT CAMSHAFT CAMSHAFT CRANKSHAFT 4. GRAY CAST IRON It is the cast iron in which most or all of the carbon is uncombined in the form of graphite flakes. Most gray cast irons are hypoeutectic alloys containing between 2.5 and 4 percent carbon. The initial appearance of combined carbon is in the cementite resulting from the eutectic reaction at 2065F<. There is experimental evidence that, with proper control of the alloying element, the alloy will follow the stable iron-graphite equilibrium diagram, ), forming austenite and graphite at the eutectic temperature of 2075F. 4. GRAY CAST IRON At any rate, any cementite which is formed will graphitize rapidly. The graphite appears as many irregular, generally elongated and curved plates which give gray cast iron its characteristic grayish or blackish fracture. As shown in figure. Graphite flakes in gray cast iron. Unetched, I0Ox 4. GRAY CAST IRON Gray iron have a different size of flakes. 4. GRAY CAST IRON REDUCING FALKE SIZE: The best method of reducing the size and improving the distribution of the graphite flakes seems to be by the addition of a small amount of material known as an inoculant. Inoculating agents that have been used successfully are metallic calcium, aluminum, titanium, zirconium, silicon carbide, calcium silicide , or combinations of these. They probably promote the nucleation of primary austenite, resulting in small grains, which reduces the size and improves the distribution of the graphite flakes. 3. GRAY CAST IRON SOME EXAMPLES: The AFS & ASTM prepared 5 flakes size which are shown in figure. 4. GRAY CAST IRON MECHAINICAL PROPERTIES: (1) TENSILE STRENGTH: Tensile strength is important in selecting a gray iron for parts that are subjected to static loads in indirect tension or bending. Such parts include pressure vessels, housings, valves, fittings, and levers, irons above 40,000 psi in tensile strength are usually considered high- strength irons and are somewhat more expensive to produce and more difficult to machine. 4. GRAY CAST IRON (2) YIELDING: Gray irons do not exhibit a well-defined yield point as do most mild steels. The stress-strain curve does not show a straight-line portion; thus a definite modulus of elasticity cannot be determined. The percent elongation is small for all cast irons, rarely exceeding 3 to 4 percent, and the reduction of area is too slight to be appreciable. (3) COMPRESSIVE STRENGHT: Compressive strength is important when the gray iron is used for Machinery foundations or supports. Like all brittle materials, the compressive strength of gray iron is much greater than its tensile strength and is largely a function of the shearing strength. Failure in compression usually occurs along an oblique plane unless the specimen is long enough to allow failure by buckling 3. GRAY CAST IRON (4) TROSIONAL PROPERTIES: Many grades of gray iron have higher torsional shear strength than some grades of steel. This characteristic, along with low notch sensitivity, makes gray iron a suitable material for various types of shafting. (5) HARDNESS: The composition also exerts a considerable effect on the hardness. Increasing carbon and silicon contents will result in decreasing hardness, although the effect is not as marked on hardness as it is on tensile strength.
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